Increasing use of digitally operating video studio apparatus, for example digital mixers, digital synchronizers, recording apparatus, and the like, require apparatus to digitize analog color video signals, that is, to convert the analog color video signals to digital form, so that they can be processed in digital apparatus. When converting analog color signals into digital form, separate coding has been found particularly desirable because this mode of coding is independent of existing analog color TV systems. Separate coding refers to an arrangement in which a color TV signal, present in accordance with any one of the known and customary systems, is separated into a brightness or luminance signal portion Y and into chrominance signal portions U, V, in which the chrominance signal portions have known characteristics. In order to mix or superimpose two digital video signals, a control is needed. The control signal can be derived in either the chroma-key or the luminance-key mode, from one of the two video signal portions.